linux-kernel - [PATCH AUTOSEL 6.12 01/31] sched_ext: selftests/dsp_local

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Message-Id: <20250218202619.3592630-1-sashal@kernel.org>
Date: Tue, 18 Feb 2025 15:25:47 -0500
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Tejun Heo <tj@...nel.org>,
	Ihor Solodrai <ihor.solodrai@...me>,
	Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.com>,
	Sasha Levin <sashal@...nel.org>,
	shuah@...nel.org,
	dvernet@...a.com,
	vishalc@...ux.ibm.com,
	linux-kselftest@...r.kernel.org,
	bpf@...r.kernel.org
Subject: [PATCH AUTOSEL 6.12 01/31] sched_ext: selftests/dsp_local_on: Fix sporadic failures

From: Tejun Heo <tj@...nel.org>

[ Upstream commit e9fe182772dcb2630964724fd93e9c90b68ea0fd ]

dsp_local_on has several incorrect assumptions, one of which is that
p->nr_cpus_allowed always tracks p->cpus_ptr. This is not true when a task
is scheduled out while migration is disabled - p->cpus_ptr is temporarily
overridden to the previous CPU while p->nr_cpus_allowed remains unchanged.

This led to sporadic test faliures when dsp_local_on_dispatch() tries to put
a migration disabled task to a different CPU. Fix it by keeping the previous
CPU when migration is disabled.

There are SCX schedulers that make use of p->nr_cpus_allowed. They should
also implement explicit handling for p->migration_disabled.

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: Ihor Solodrai <ihor.solodrai@...me>
Cc: Andrea Righi <arighi@...dia.com>
Cc: Changwoo Min <changwoo@...lia.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/testing/selftests/sched_ext/dsp_local_on.bpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c b/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c
index c9a2da0575a0f..eea06decb6f59 100644
--- a/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c
+++ b/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c
@@ -43,7 +43,7 @@ void BPF_STRUCT_OPS(dsp_local_on_dispatch, s32 cpu, struct task_struct *prev)
 	if (!p)
 		return;
 
-	if (p->nr_cpus_allowed == nr_cpus)
+	if (p->nr_cpus_allowed == nr_cpus && !p->migration_disabled)
 		target = bpf_get_prandom_u32() % nr_cpus;
 	else
 		target = scx_bpf_task_cpu(p);
-- 
2.39.5